Filter elements disclosed in DE 4312705A1 serve generally to free polluted fluids of pollutants, especially fluids in the form of hydraulic power oil. Pollutants are intercepted by the mat filter of the filter element, and are retained and held out of the fluid current. When the mat filter is filled completely with pollutants, it is exchanged for a new mat filter or the complete filter element is replaced with an entirely new filter element. In this device, the polluted fluid passes through at least one of the two end caps through a corresponding inlet opening into the filter element, and flows through this element for a purification process occurring from the outside inward. For this purpose, the mat filter is arranged between the supporting pipe with openings and the filter casing likewise provided with openings. In order to attain a high pollutant intercepting capacity, the mat filter is pleated with a plurality of pleats, in other words is arranged in folds around the supporting pipe, the other end cap can be provided with a safety or bypass valve, and can allow the fluid current to bypass the filter element insofar as the mat filter no longer allows fluid penetration because it is completely clogged with pollutants.
In the filter element disclosed in DE 4312705 A1, the cylindrical filter casing which surrounds the mat filter is formed of an expanded metal fabric. The two ends of the casing are bent toward one another and inward to open in a retaining clip, which forms the foundation for a layer of adhesive. The adhesive in this case is a two-component adhesive. Because of the plurality of manufacturing stages and the accompanying structural components, the known filter element is costly to manufacture. Furthermore, there are maintenance problems with such filter elements when they become unusable, especially in relation to the expanded metal casing of metal fabric. Final maintenance of the filter element, for example when the filter element is to be processed as an entirety in a suitable shredder unit, is consequently not possible and limits the recycling capacity to individual components of the filter element.
GB-A-2,007,992 discloses a filter element having a filter casing formed from a flat blank. The blank ends are turned toward one another with formation of a filter chamber, and are tightly joined together by means of a sealing seam produced by a heat-sealing method. In order to attain tightly fitted contact of the mat filter with the interior supporting pipe and the cylindrical filter casing, so that even with the stresses of exchange the mat filter is supported securely in the filter element, a shrinking process is applied to the exterior cylindrical casing with exterior heat application. However, this application can lead to detrimental thermal stresses for the mat filter.
A filter element having an interior and/or exterior supporting pipe and frontal end caps is disclosed in GB-A-1,411,607. The supporting pipe is formed of a flat blank of plastic. The overlapping blank ends are bent toward and welded with one another with formation of a sealing seam. The filter material in the form of filter paper is rolled up, forming a hollow cylinder that can be inserted in the filter housing. In this case tolerances occur which interfere with a secure supporting of the mat filter. This construction reacts as sensitive to the stresses of exchange.
DE-A-19538185 discloses a filter element for gases or liquids with folded filter material arranged in the shape of a ring, penetrable from the exterior to the interior and arranged between an interior perforated cylindrical support body and an exterior perforated cylindrical casing. The cylindrical casing is produced by bending a perforated flat material. The material edges are subsequently connected with each other. For the production of this filter element, the perforated flat material in the form of a plastic strip is wound helically into the form of a cylinder. The helically running edges are attached to one another by a plastic sealing compound. Here too the ring of star-shaped folded filter material arranged between the interior cylindrical support body and the exterior cylindrical casing is arranged with some spacing, which in turn heightens the sensitivity in relation to stresses arising from exchanges.